The present invention relates to a lighting power source and a light fixture for powering one or more lamps or light sources such as a light emitting diode (LED). More particularly, the present invention relates to a lighting power source having control circuitry for reducing the charging time in a driving capacitor, and an associated light fixture for mounting the lighting power source.
An example of a lighting power source as conventionally known in the art is as shown in FIG. 14. A diode bridge DB is provided as a DC power source that outputs pulsating DC power) by full-wave rectifying an AC power input from an AC power source AC. A first switching element Q1 (e.g., an N-channel type MOSFET) is connected at a first end to a high voltage output side of the diode bridge DB. A diode D1 is connected across the output ends of the diode bridge DB, with its anode coupled to the low voltage output side and its cathode coupled to high voltage side opposite the first switching element Q1. An output circuit includes an inductor L1 forming a loop in conjunction with the diode D1 and to which a load Z is connected. A control circuit 2 is provided which is effective to turn on/off the first switching element Q1.
In the conventional lighting power source 1 as shown in FIG. 14, the output circuit is a boost converter (a step-up chopper circuit) including a series circuit of the inductor L1 and a step-up switching element Qb that is connected in parallel with diode D1, and a series circuit of a step-up diode Db and a capacitor C1 that is connected in parallel with the step-up switching element Qb. More specifically, both ends of the capacitor C1 are connected as outputs to load Z, and DC power across the capacitor C1 is output to the load Z. The first switching element Q1, the diode D1, the inductor L1 and the capacitor C1 collectively define a buck converter (a step-down chopper circuit). The control circuit 2 includes a switch driver circuit 2a for turning on/off the first switching element Q1, and a feedback circuit 2b that controls the switch driver circuit 2a to maintain a constant voltage across the capacitor C1 (i.e., an output voltage of the lighting power source 1), and also turns on/off the step-up switching element Qb.
The conventional lighting power source 1 shown in FIG. 14 includes a diode D1 and the output circuit interposed between the output end at the low voltage side of the diode bridge DB as the DC power source and a terminal at the low voltage side of the first switching element Q1. Therefore, a driving capacitor Cs, which is connected at one end at the low voltage side of the first switching element Q1 (i.e., between the first switching element Q1 and the diode D1), is provided as the power source for the switch driver circuit 2a of the control circuit 2 to drive the first switching element Q1. Moreover, a charging capacitor Cc is provided as a charging power source that is connected to the other end of the driving capacitor Cs and is supplied with power from the diode bridge DB via a resistor Rc to charge the driving capacitor. More specifically, the voltage at one end at the low voltage side of the first switching element Q1 is approximately equal to the voltage at an output end at the low voltage side of the diode bridge DB during a period in which current flows through a loop defined by the diode D1 and the output circuit, such that the driving capacitor Cs is charged by current supplied from the charging capacitor Cc via the charging diode Dc. Then, the current for charging the driving capacitor Cs flows through a circuit loop including the inductor L1.
The control circuit 2 in FIG. 14 further includes a timer circuit 2c that measures a predetermined charging time after turning the power on, and turns on the step-up switching element Qb by an output via an OR circuit OR1 during the measurement of the charging time. More specifically, the driving capacitor Cs is charged by current flowing via the inductor L1 and the step-up switching element Qb during a period in which the step-up switching element Qb is turned on by way of the above-mentioned operation.
When the driving capacitor Cs, which is not charged when, for example, the power source is just turned on, is charged via a pathway through the inductor L1 as described above, it can be considered that because of the inductor L1, it takes a relatively long time to sufficiently charge the driving capacitor Cs, and it becomes difficult to control the voltage across the driving capacitor Cs.
A technique is further known in the art wherein a switch (not shown) for selectively generating a short circuit across the diode D1 is provided, and the switch is turned on before the control circuit 2 starts turning on/off the first switching element Q1 and during the period in which the voltage across the charging capacitor Cc is below a predetermined value. Because such a configuration can realize the charge via the above-mentioned switch and not through the inductor L1, the voltage across the driving capacitor can become stable in a relatively short time.
Here, the case where PWM control is carried out will be considered. More specifically, the control circuit 2 performs either an operation during an ON period of repeatedly turning on/off the first switching element Q1 to maintain a constant voltage across the capacitor C1, or an operation during an OFF period of keeping the first switching element Q1 in an OFF state, according to an input PWM signal. In this case, the higher a ratio of the ON period in one period (i.e., the higher the on-duty), the more the output power increases.
Conventionally, the switch for causing a short circuit across the diode D1 has been turned on during a period in which the voltage across the charging capacitor Cc is below a predetermined value, i.e., only immediately after turning on the power source (namely, immediately after a DC power source E starts outputting the DC power). Therefore, when the PWM control as described above is carried out, a decrease in voltage across the driving capacitor Cs during the OFF period may make it impossible to turn on the first switching element Q1 when the next ON period starts.
Further known in the art is a technique wherein a switch (not shown) is provided for selectively causing a short circuit across the diode D1, and the above-mentioned switch is turned on before the control circuit 2 starts turning on/off the first switching element Q1 after turning on the power source. More specifically, the charge can be realized via the above-mentioned switch and not through the inductor L1, such that the voltage across the driving capacitor Cs can become stable in a relatively short time.
However, a sudden surge of current immediately after turning on the above-mentioned switch may put an excessive electric stress on circuit components such as the driving capacitor Cs and the charging diode Dc defining the charging pathway for the driving capacitor Cs.